Many commercial processes involve mixing of fluids, including especially catalytic reactors and large fractionation columns. Such mixing is not always a simple matter, especially where the fluid has multiple phases (such as liquids and gases/vapors), and where large volumes are being rapidly mixed. Numerous mixing apparatus are known, and some of these are described in U.S. Pat. No. 6,098,965 to Jacobs et al. (August 2000), which is incorporated herein by reference in its entirety. Jacobs et al. teach several improvements, some of which involve bubble caps spaced apart on a distribution plate.
Bubble caps generally comprise a riser and a cap, arranged such that a fluid flows upwards in a space between the cap and the riser, reverses direction and then flows downward through a passageway in the riser. In the absence of swirl directors, the fluid flow path is thus generally in the shape of an inverted “U”. Bubble caps are generally affixed to a distribution plate, and the passageway through the riser is confluent with a hole in the distribution plate. Bubble caps often contain a plurality of side slots that provide an entrance for the gas phase into the annular space between the riser and the cap. The gas entrains liquid present in the annular space. See, for example, U.S. Pat. No. 5,158,714 to Shih et al. (October 1992), which is herein incorporated in its entirety by reference.
There must be some mechanism for maintaining the position of the riser with respect to the cap. It is known to use cantilevered arms or other spacers for that purpose. See, for example, U.S. Pat. No. 5,989,502 to Nelson et al. (November 1999) and U.S. Pat. No. 4,305,895 to Heath et al. (December 1981), each of which is incorporated herein in its entirety by reference. In the past, such spacers have always been of minimal size to reduce cost and minimize any flow effects. Prior art spacers therefore exclusively serve a positioning function, and do not materially assist in either fluid flow or mixing.
Skirt height has been shown to materially affect the fluid flow and mixing. See, for example, “Optimum Bubble-Cap Tray Design”, Bolles, William L., a four part series in Petroleum Processing, Vol. 11, No. 2, pp 65–80; Vol. 11, No. 3, pp 82–95; Vol. 11, No. 4, pp 72–79, Vol. 11, No. 5, pp 109–120, which is incorporated herein in its entirety by reference. In this series of articles, Bolles presents a design methodology for bubble caps of the type commonly used in distillation columns. In such columns, the vapor flow is upward through the bubble cap tray and the liquid flow is transverse, across the bubble cap tray. Such flow is typically described as countercurrent flow. In the Bolles article, at Vol. 11, No. 3, p. 87, a skirt height of 1.3 cm to 3.8 cm is recommended, and there is a suggestion that greater skirt heights would be disadvantageous. There is certainly no teaching, suggestion, or motivation of which the current applicants are aware, for skirt heights greater than 3.8 cm.
Conversely, Ballard et al. (U.S. Pat. No. 3,218,249) teaches the use of bubble caps as a mixing and distribution means for the concurrent downflow of vapor and liquid. Ballard et al. teaches skirt heights of any distance “ . . . above the distribution tray so long as the flow of gas through the downcomers is not sealed off; a reasonable range being from a level corresponding to practically no distance above the tray to a distance of about one foot thereabove.” Ballard et al. further teaches that “ . . . the liquid phase, disengaged from the vapor phase by gravity, fills up on tray 18 to a level below the slot depth in the downcomer caps, such level being determined primarily by the gas flow rate per cap. It is, of course, necessary that some of the slot openings be exposed above the liquid surface to permit passage of vapor therethrough. Where the caps have no slots, the liquid level on the tray will be below the bottom rims of the caps for the same reason. Where unslotted caps are used, clearance between the bottom rim and the tray must be maintained to accommodate the passage of gas and liquid thereunder.” Clearly, the skirt height dimensional range taught by Ballard, et al. applies specifically to an unslotted cap, because vapor flow through a slotted cap can not be blocked off by reducing the skirt height to practically no distance. There is no teaching of a specific dimensional range suitable for slotted bubble caps.
Shih, et al. (U.S. Pat. No. 5,158,714) teaches the use of a dispersion plate to improve the distribution of liquid exiting the riser. Gamborg, et al. (U.S. Pat. No. 5,942,162) teaches the use of a slotted bubble cap, modified such that the cap is non-concentric with the riser, to improve the uniformity of liquid distribution. Gamborg, et al. describe this modified bubble cap as a vapor lift tube, wherein the cap is called an upflow tube and the riser is called a downflow tube. Nonetheless, the fluid flow path is the shape of an inverted “U”, flowing first upward through the upflow tube and then downward through the downflow tube. Jacobs, et al. (U.S. Pat. No. 6,098,965) teaches the use of riser vanes and/or target plates to improve the distribution of liquid exiting the riser. Aside from the patents cited above, the current applicants are not aware of any other information in the public domain that discloses technological advances in the use of bubble caps as a mixing and distribution means for the concurrent downflow of vapor and liquid.
Some systems that utilize bubble caps provide for rough distribution of fluids upstream of the bubble caps. A patent granted to Stangeland, et al. (U.S. Pat. No. 5,690,896 November 1997) describes an apparatus for rough distribution comprising a perforated plate located directly above the bubble cap tray. With this approach, the perforations must pass both the gas phase and liquid phase fluids. As a result, the prevailing liquid level on this tray may be quite low, thereby negatively impacting the quality of rough distribution. A patent granted to Grott, et al. (U.S. Pat. No. 5,837,208 November 1998) describes an apparatus for rough distribution consisting of a perforated tray surrounded by cylindrical wall. With this approach, the gas phase fluid can flow through the annular area between the perforated tray and the reactor wall, while the liquid phase fluid flows primarily through the perforations. One drawback of this approach is that the annularly downflowing gas phase fluid can disturb the liquid surface on the bubble cap tray, thereby negatively impacting the performance of the bubble cap tray. Finally, with both of the above approaches, the perforated trays restrict inspection and maintenance access to the bubble cap tray.
Thus, there is still a need for improved methods and apparatus for mixing and distributing fluids, including improvements to bubble cap trays and rough distribution mechanisms.